PROJECT SUMMARY/ABSTRACT In this project we will seek a better understanding of why and how mouse strains lacking the NF?B RelB protein, known as the primary effector of the non-canonical NF?B pathway, develop severe inflammatory and auto-immune disease. Whereas previous studies focused on NF?B response genes, our unbiased transcriptomic measurements (preliminary results) revealed that RelB-deficient macrophages and dendritic cells show dramatic hyper-expression of interferon stimulatory genes (ISGs) due to hyper-expression of interferon-?. Hyper-activity of the type I IFN regulatory system in antigen-presenting cells may indeed explain the T-cell mediated auto-immune phenotype in RelB knockout mice. The proposed project addresses the overarching hypothesis that RelB functions as a critical signaling node that fine-tunes inflammatory and interferon-mediated responses during the transition from innate to adaptive immunity. In the first Specific Aim, we will first characterize the control of interferon gene expression by RelB in macrophages and dendritic cells, not only in RelB knockouts but also naturally occurring splenic DCs that show either low or high expression of RelB (using a novel RelB-Venus reporter). We will examine whether reducing the hyper-activity of the interferon system genetically will suppress the auto-immune phenotype of RelB-/- mice. In the second Specific Aim, we will characterize the mechanism of how RelB regulates type I interferon responses. Using a novel RelB-DNA binding mutant (RelBdb/db), we will distinguish between chromatin-bound vs. cytoplasmic mechanisms. Preliminary results suggest that (i) hyper-activation of IRF3 may be mediated by cytoplasmic inhibition by RelB of RelA, either by direct binding or via stabilizing the RelA-trapping I?Bsome, and that (ii) RelB:p50 may directly inhibit IFN? expression by competing with IRF3 for binding to the G-IRE (Cheng et al 2011). These mechanisms will be delineated in the proposed studies.